1. Field of the Invention
The present invention relates to a steel product with heat-resistant, corrosion-resistant plating layers, and more particularly, to a steel product such as sheets, pipes, couplings, clamps, bolts, and-nuts used for automobiles and various machines and equipment, said steel product having heat-resistant, corrosion-resistant plating layers on the surface thereof.
2. Description of the Prior Art
It has been a common practice to form a zinc plating layer and then a chromate film on sheets, pipes, couplings, clamps, bolts, and nuts used for automobiles and various machines and equipment. However, it has been found that the zinc plating alone does not meet the requirement for these components which has become severer than before. For the improved corrosion resistance, the zinc plating has been superseded by Sn-Zn or Zn-Ni alloy plating or a combination of this alloy plating and Zn plating. In this connection, there is disclosed in Japanese Patent Laid-open No. 165387/1985 a corrosion-resistant steel pipe with plating layers composed of an electroplated Zn-Ni alloy layer (on the outside of the steel pipe), an electroplated zinc layer (on the Zn-Ni alloy layer), and a chromate film (on the zinc layer).
The above-mentioned plated steel products exhibit improved corrosion resistance in corrosive environments at normal temperature but they do not in high-temperature environments such as automotive engine rooms.